1. Field of the Invention
The present invention generally relates to high resolution photolithography and, more particularly, to improved fabrication methods for high performance masks for use therein.
2. Description of the Prior Art
The formation of fine patterns of conductors and other portions of circuit elements is an indispensable part of the fabrication of integrated circuits and other electronic devices, such as multi-layer modular circuits which may contain many such integrated circuits and other devices which are connected by conductive patterns on lamina thereof. Photolithography techniques are well-known and highly developed for the production of such patterns. In general, photolithography involves the application of a photosensitive resist material to a surface of a lamina, substrate or partially formed integrated circuit and the exposure of a portion of the photosensitive resist material in accordance with a desired pattern. The pattern is then developed by selective removal of either the exposed or unexposed portion of the resist (depending on whether the resist material is a positive or a negative resist) allowing material to be selectively removed or deposited in accordance with the remaining pattern of resist material.
The exposure of the photoresist material is often accomplished by the projection of light or other radiation (e.g. at ultra-violet and shorter wavelengths) through a mask since a mask permits a high degree of accuracy, repeatability and convenience as compared to direct writing of the pattern. The quality of the mask therefore determines and limits the quality of the developed pattern of resist material. While very high quality mask patterns have been developed, however, some optical effects have further limited resist exposure quality.
Specifically, due to the wave-like nature of light and other forms of radiation suitable for photolithography processes, diffraction and other interference effects occur at the edges of opaque areas of the mask and may cause a dimensional change (or produce ghost patterns) in the exposed pattern since the opaque regions of the mask cannot be placed directly in contact with the photo resist during exposure. In practice, it is customary to project the image using an optical system of substantial length in order to achieve a reduction of the size of the pattern at the resist surface relative to the size of the mask. These effects therefore cause some spreading of the exposed image or even the exposure of additional regions of the photoresist corresponding to lobes of energy radiating at an angle to the plane of the mask from an aperture therein, depending on the separation of the opaque regions of the mask from the photoresist (e.g. the distance over which the pattern image is projected).
While this effect is generally dimensionally small, recent increases in integration density of integrated circuits has pushed minimum feature sizes of patterns into regimes where the effect has become significant and often critical to high manufacturing yields. Therefore, to improve exposure patterns, a so-called rim phase-shift mask has been developed in order to limit image spreading in exposure of features of closed shape. A similar phase-shift mask formation known as a Levenson-type shifter is used for exposure of periodically repeated patterns, such as arrays of parallel conductors.
Essentially such rim phase-shift masks provide an altered optical path length through the mask at the edge or rim of the opaque pattern formed in the mask. A Levenson-type phase shift structure provides a similar effect with differing path lengths between opaque regions which enhances contrast of repeated patterns, such as parallel lines. The difference in path length provides a 180.degree. phase shift of the radiation at the wavelength at which the exposure is made. This phase shift causes an interference effect which slightly narrows the exposure pattern at the photoresist surface relative to the size of an aperture formed in the mask and reduces the intensity of radiation beyond the edges of the aperture (e.g. the energy in the side lobes) such that any exposure which occurs beyond the dimensions of the mask aperture is insufficient to be developed.
While rim phase-shift masks have been made and effectively used, the fabrication of the masks has been difficult and expensive due to the need to form extremely small regions having differing optical lengths at the edges of opaque regions. That is, either patterning must be done within the mask pattern or the opaque regions of the mask must be recessed from the regions of differing optical path length.
For example, one known technique, which will be discussed in greater detail below, involves selective etching of the mask substrate in addition to the etching of the mask pattern. This requires two separate exposure and development operations which are therefore impossible to conduct in a self-aligned manner, thus increasing expense and increasing the likelihood of production of a defective mask. Further, the requirement for forming a pattern within another pattern limits the minimum feature size which can be exposed by such masks. For example, to form a 0.4 .mu.m exposure spot using a 4.times. mask (e.g. the mask is formed at four times the size of the desired image in each coordinate direction), it is preferred to provide phase shift regions of about 0.5 .mu.m in addition to the 1.6 .mu.m clear aperture. Therefore, even with optical reduction in size of the exposure pattern defined by the mask, a comparable level of photolithographic technology is still necessary to form the mask. By the same token, the size of openings in opaque regions in the mask are increased and spacing between exposed regions cannot be reduced to exploit the full capability of the currently available photolithographic technology in terms of integration density of the final product.
Another known technique which will also be discussed in greater detail below, involves exposure of a phase shift material (e.g. a thickness of material having similar index of refraction to the mask substrate and of a thickness to provide the desired phase shift of radiation passing therethrough), which is also a photoresist, from the rear surface of the mask in order to form the required pattern of phase shift material. While this exposure can be considered to be self-aligned, the opaque material of the mask must then be undercut by an etching process which may not be accurately controllable. That is, the final pattern of opaque material (e.g. chromium) will not have been directly formed in accordance with a desired final pattern and difficulty of control of the recess distance and the sidewall profile of the opaque material may cause undesired variation from the intended final pattern or result in the formation of a defective mask.
In regard to either of these techniques, it should be noted that many patterns and most interconnetion patterns which are used in the production of integrated circuits will include both repeated patterns of features such as parallel conductors as well as closed features such as pads. Heretofore, the process steps required for Levenson-type phase shift structures was sufficiently incompatible with the formation of rim phase shift structures that separate alignment, exposure development and etch steps were required for the respective structures. Therefore, increased costs and reduced manufacturing yields resulted because of the additional alignment steps and the additional process steps required.
In summary, known techniques for fabricating masks including rim phase-shift structures require some operations that inherently carry a risk that a defective mask will be formed. Further, all known techniques limit the resolution which can be achieved in the final product formed with the mask below the resolution which can be produced at the current state of the photolithographic art as well as requiring numerous and expensive steps for mask fabrication.